In the meantime I have been able to sketch the input stages of 2i2.
Attached is the block diagram.( If the need arises I can upload the sketch. ) After studying the same and noting that the first stage of line amp is a unity gain buffer,how much would be the error if the gain of the mic preamp is entered as the left channel preamp gain entry in ARTA ?

In the meantime I have been able to sketch the input stages of 2i2.
Attached is the block diagram.( If the need arises I can upload the sketch. ) After studying the same and noting that the first stage of line amp is a unity gain buffer,how much would be the error if the gain of the mic preamp is entered as the left channel preamp gain entry in ARTA ?

Success now depends on where ASIO inputs/outputs are connected in this diagram.

The challenge surpasses my intellectual ability. I guess I will have to go with an external mic pre or use another measurement software unless creators of ARTA come up with a solution.
Thanks for your time.

You can probably go along with single-channel FFT mode measurements. My observations are that with linear card it doesn't give significant errors compared to dual-channel FFT. In this mode you don't need to have extra input and dual-channel FFT related calibration.